GM_addStyle is usually for beginners and a shortcut to adding it by hand to the head tag... so once you determine what node in the DOM (the web page source) you can do something like body { color: white !important; background-color: black !important; } ... or in your case the rules would be { width: 8000px !important; } and { height: 8000px !important; }.

Since I am not sure where you want this in the DOM exactly you'll have to find the selector.

That's because you didn't do it with GM_addStyle as I linked above... you'll need to @grant that API function too in the UserScript metadata block in order to use it. A more advanced technique is getting the head tag and inserting a style tag in yourself in which case you wouldn't need the @grant which sandboxes your scripts usually.